1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory having an electric charge accumulating insulating film made of a high dielectric material, and a method of manufacturing the same.
2. Description of the Related Art
Generally, a flash memory is known as one of nonvolatile semiconductor memories, and it is widely used in electronic appliances. This flash memory does not require any electric holding action for storage (holding power supply), and programs can be easily written therein after the product is completed.
A further finer structure and operation at a lower voltage are demanded in next-generation and future NAND flash memories. For the NAND flash memory, a flat-cell structure is needed because of difficulty in burying IPD (inter-poly dielectrics) in cells in the manufacturing process. To realize the flat-cell structure, in addition to assurance of charge amount by compensating for decrease in contact area of IPD and FG (floating gate), it is essential to reduce the thickness of IPD, FG and a tunnel film in order to suppress interference between cells and lower the voltage. In the FG structure, to maintain the nonvolatility, the tunnel film must be thicker than specified, which is contradictory to realization of the fine structure. This is because, in the case of the FG structure for accumulating an electric charge in a metal gate film, if a local defect occurs in the tunnel film, the majority of the accumulated charge is lost through the defect. One of the methods for solving the problem is to use a memory cell structure having a discrete type charge accumulating layer instead of the FG structure. One of the candidates therefore is MONOS (metal-oxide-nitride-oxide semiconductor).
The structure of a memory cell containing the charge accumulating layer by conventional MONOS is composed of, for example, a multi-layer structure of a tunnel layer of an insulating film (oxide film), a trap layer of a silicon nitride film, a block layer of an insulating film, and a metal gate electrode formed on a channel region of a silicon substrate formed with a source and a drain.
This MONOS is formed by accumulating an electric charge on silicon nitride (SiN). In the technologies disclosed so far, at practical level, the charge accumulating amount of silicon nitride is not enough, and the threshold voltage change width is not sufficient. If the silicon amount is increased in order to increase the charge accumulating amount, for example, the state become metallic, and the same problem as in the conventional FG structure occurs.
Further, when silicon nitride is used in the charge accumulating layer, a sufficient driving voltage must be applied to the tunnel film, and low-voltage operation is not expected. This is because the dielectric constant of silicon nitride is low. Besides, when erasing data, the technology using sufficient threshold change width by drawing out electrons excessively cannot be applied to the charge accumulating layer of silicon nitride. This is mainly because a great energy is needed for drawing out electrons further after reaching a state free of writing by electrons. Furthermore, since the dielectric constant of a silicon nitride film is low, the charge capturing section area is smaller. As a result, the charge capturing efficiency is poor, and the threshold controllability is low.
It has been also attempted to fabricate a charge accumulating film by exposing TiO2, instead of silicon nitride, to plasma damage. In this case, since lots of oxygen defects occur, an electric charge is accumulated, but oxygen defects in ionic oxide generally have a property to generate a state near the conduction band bottom. Accordingly, when a charge accumulating layer is formed by damaging ionic oxide such as TiO2 or HfO2, the accumulating layer behaves like an n-type semiconductor, and the charge may be lost by local defect of the tunnel film. That is, a charge accumulating layer making use of oxygen defects such as TiO2 may have an imperfect structure, in principle, in terms of holding of electric charge. To solve this problem, a nonvolatile semiconductor memory device is proposed, for example, as disclosed in JP-A 2004-336044 (KOKAI).
This publication, JP-A 2004-336044 (KOKAI) discloses a technology of introducing Lantanoide elements into HfO2, ZrO2, TiO2, as the charge accumulating layer. For example, in addition of La (exactly the same in other Lantanoide substances), La is a trivalent substance additive, and a substance of valence of +3 is introduced instead of valence of +4, and oxygen is deficient and is stabilized in order to compensate for electric charge. This oxygen defects behaves like n-type and much charge cannot be collected, and it is difficult to realize high density charge trapping by introduction of La.